Support environments API

[ryepup]

From the Environments docs:

Environments allow control of the continuous deployment of your software, all within GitLab.
...

• Provides a full history of your deployments for each environment.
• Keeps track of your deployments, so you always know what is currently being deployed on your servers.

API endpoints are available on https://docs.gitlab.com/ee/api/environments.html

[Xhanti]

Ran into this today, I need to invoke the stop method for gitlab environments and this client doesn't support it. I see this PR: #240 add support for it. Is this something you are considering adding?